A new era in web browsing was kicked off by the European Commission on Monday, as Microsoft was forced to allow users of its Windows operating system to choose different programs to surf the internet.

The effect will certainly be to broaden competition in the market – which should boost innovation, says Opera, the Norwegian company which filed the original complaint that led to the action.

From this week, users in the UK and the rest of Europe who use Windows XP, Windows Vista or Windows 7 and have Microsoft's Internet Explorer (IE) as their default browser will be sent to a screen offering a choice of 12 browsers, including IE. The top five by market share – IE, Mozilla's Firefox, Google's Chrome, Apple's Safari and Opera will be presented first, in random order.

The so-called "ballot screen" follows the settlement of an anti trust case by the European Commission, following a complaint filed by Opera in 2007. That alleged that Microsoft was illegally bundling its own browser into Windows, giving IE an unfair advantage, because users have to actively download other browsers. The claim is similar to that which led Microsoft to be prosecuted by the US Justice Department in the late 1990s.

But where the US later settled the case without any substantial penalty for Microsoft, the EC has taken more direct action.

Jon von Tetzchner, the co-founder and chief executive of Opera, told the Guardian that the move would certainly give Opera more users, and would be good for the web as a whole.

"We will see people move away from Internet Explorer," he said. "It's very clear that when people are offered a choice of browsers, they will try others out and will choose to use others [rather than IE]."

IE dominates the browser market because of its inclusion in Windows, the most widely-used PC operating system.

The versions of IE released since 2000 have about 62% of the market. Firefox has 24%, with Chrome, Safari and Opera having around 5%, 4% and 2.5%.

Microsoft's browser has been the target of criticism for its failure to adopt emerging standards — and of repeated attacks by hackers who realised that because of its wide usage, any flaw they exploit will affect millions of users.

The main beneficiaries of the ballot are likely to be Google, Apple and the Mozilla Foundation, the makers respectively of Chrome, Safari and Firefox.

It's probably not a "phishing attack": Microsoft Windows users in Europe are now starting to get a screen that offers them a choice of web browsers, as a result of an anti-trust settlement with the European Commission.